1. Field of the Invention
This invention pertains generally to the application of a coating onto fibers or flat substrates, and more particularly to an improved method and system for generating spallation of ultra thin films and measuring the interface tensile strength between a substrate and the thin film.
2. Description of Related Art
Films and coatings have widespread use in different industries. Examples include thermal barrier coatings for engines; tribological coatings in cutting tools, seals, and joints; polymeric layers in paint assemblies; fiber coatings in composites; electrical, magnetic and optical multilayers in electronic devices; metal and ceramic films in MEMS-based mechanical and clinical devices, among others. In the field of composite materials, the interface between a thin coating and a fiber is considered for deflecting impinging matrix cracks. In the field of tribology, interfaces between various types of functional coatings, e.g., magnetic, conducting, optical, or electrical, protective coatings, e.g., thermal barrier, corrosion, or wear resistant, or decorative coatings and their underlying substrates are of interest.
In the foregoing various applications, the tensile strength of the interface is an important property that directly controls the interface decohesion process, and often controls the usefulness and reliability of the coating component. Adhesion of films is a prerequisite to carryout their intended functions. Thus, the central goal in all these applications is to avoid film delamination and coating failures by maximizing adhesion, and to predict long-term reliability of the coated components. Additionally, the measurement of the interface tensile strength is of importance for reliable performance of the coating in the above applications. These issues are usually addressed by seeking a fundamental understanding of the adhesion between different layers as a function of process (film deposition and surface variables) and service (moisture and temperature) variables.
These objectives are currently accomplished using adhesion metrology tools, such as the laser spallation technique commonly applied in the art. Typically, a high energy laser pulse is made to impinge upon a planar arrangement of a confining plate, a metallic layer, a substrate plate, and a coating combination. The laser pulse impinges on a thick metal film that is sandwiched between the back surface of a substrate of interest and a fused quartz confining plate that is transparent to the wavelength of the laser. Normally, gold or aluminum is used as the laser absorbing film. Absorption of the laser energy in the confined gold leads to a sudden expansion of the film which, due to the axial constraints of the assembly, leads to the generation of a compressive shock wave directed towards the test coating interface. A part of the compressive pulse is transmitted into the coating as the compression pulse strikes the interface. It is the reflection from the free surface of the coating of this compressive pulse into a tension pulse that leads to the removal of the coating, given a sufficiently high amplitude.
U.S. Pat. No. 5,438,402 provides significant improvement in the art to determine the tensile strength of planar interfaces down to 1 micrometer in thickness. However, ultra-thin layers with thickness below 0.5 micrometers are now the focus of research in the microelectronics industry for developing ultrahigh density devices using nanotechnology. In addition, adhesion at similar length scales will become important in the material optimization of MEMS-based mechanical and clinical devices when they are mass-produced. Therefore, there is a need to extend measurement capabilities to films below 0.5 μm in thickness.
Accordingly, an object of the present invention is to measure the tensile strength of interfaces between such very thin films.
Another object is to separate and lift thin film lines or their complete structures from semiconductor and engineering substrates using glass-modified stress waves, and catch them on desired substrates for reconstructing structures. This can lead to a faster way of fabricating MEMS and nano-scale devices by bypassing the currently used wet-etching techniques. At least some of these objectives will be met in the following invention.